1. Field of the Invention
The present invention relates to a two-stroke internal combustion engine, and in particular to a two-stroke internal combustion engine which is suited for use in a portable working machine, such as a chain saw, that is adapted to be operated in various handling postures.
2. The Prior Art
An air-cooled two-stroke gasoline engine of small in size is generally constructed such that an air/fuel mixture is sucked via an inlet port into a crank chamber, in which the air/fuel mixture is pre-compressed and at the same time agitated by a balance weight rotating in the crank chamber. In such engines, a portion of unatomized raw fuel in the sucked air/fuel mixture is sometimes unintentionally fed via a scavenging passage into the combustion chamber. This phenomenon can be seen more frequently when the engine is in a state of idling.
When the engine is in a state of high load and high rotational speed, the quantity of fuel per unit time is relatively large. In this state, even if the unatomized raw fuel flows in a rush into the combustion chamber, no serious inconvenience results. However, when the engine is in a state of idling, the quantity of fuel per unit time is relatively small. If, in such circumstances, the unatomized raw fuel is allowed to flow in a rush into the combustion chamber, the air/fuel mixture becomes excessive in concentration, and results in a serious problem, e.g. stalling and stoppage of the engine.
An air-cooled two-stroke gasoline engine of small in size (hereinafter referred to simply as an internal combustion engine or an engine) which is commonly employed as a power source for a portable working machine to be operated in various handling postures, such as a chain saw, is generally arranged so as to be oriented horizontally in a main case or housing during use. The engine is typically elongated, i.e. larger in the longitudinal direction (height) than in the lateral direction. Due to this structure of the internal combustion engine, the aforementioned phenomenon becomes more prominent, resulting in the sudden stalling and stoppage of the engine.
In the case of the aforementioned small two-stroke internal combustion engine which is adapted to be disposed horizontally in a main case of a working machine, such as a chain saw (as disclosed, for example, in Japanese Utility Model Unexamined Publication S/56-140402), a suction port is formed at the bottom (a side portion when viewed in a horizontal posture) of the crank chamber and an air/fuel mixture from a carburetor is sucked from the suction port via a reed valve to the crank chamber to be pre-compressed therein, the resultant pre-compressed air/fuel mixture then being transferred through a scavenging passage to the combustion chamber. Heretofore, there has frequently been experienced a phenomenon of sudden stalling of the engine, resulting in the stoppage of engine, when the forward portion of the working machine (e.g., chain saw) is directed upward or obliquely upward after the working machine has been operated with the forward portion thereof directed downward or obliquely downward for a period of time.
The cause of this phenomenon can be explained as follows. Namely, an unatomized raw fuel (a liquid fuel) which has been adhered or collected at first on the inner peripheral wall of the crank chamber of the engine collectively flows into the suction port to be accumulated therein during a period when the forward portion of the working machine is directed downward or obliquely downward, i.e. when the suction port which opens to the crank chamber of the engine is directed downward or obliquely downward. When the forward portion of the working machine is thereafter directed upward or obliquely upward, the unatomized fuel that has been accumulated in the suction port flows into the scavenging passage along the inner peripheral wall of the crank chamber and then flows in a rush into the combustion chamber from the scavenging passage, thereby supplying an excessively concentrated air/fuel mixture to the combustion chamber for combustion.
In an attempt to prevent this undesirable flow of unatomized raw fuel in the crank chamber due to the change in handling posture of engine, the assignee of the present invention have proposed in Japanese Patent Application H/7-313371 the installation of a flow control portion or structure at the inner peripheral wall of the crank chamber.
According to this proposal, a flow control portion or structure consisting of one or two dams, grooves or recesses is installed near the inlet port of the crank chamber for interrupting the flow of the unatomized raw fuel, thus preventing the unatomized raw fuel from entering into the combustion chamber even if the unatomized raw fuel accumulated at the inlet port is forced to flow into the crank chamber due to a change in handling posture of the engine.
However, since the dams and the like functioning as a flow control structure are formed in the vicinity of the inlet port in parallel with the crank shaft according to the aforementioned proposal, it has heretofore been difficult to prevent the inflow into the combustion chamber of unatomized raw fuel that adheres to an inner wall portion of the crank chamber on the scavenging passage side of such dams or flow control structure before the engine is changed in handling posture, or the inflow into the combustion chamber of the unatomized raw fuel that passes over the dams or flow control structure on the occasion of change in handling posture of the engine.